Soft contact lenses made from silicone hydrogels offer improved oxygen permeability as compared to soft lenses made from non-silicone materials such as poly(2-hydroxyethyl methacrylate) (HEMA). Initial efforts to make silicone hydrogel contact lenses were hampered by the poor wettability, high modulus, poor clarity, hydrolytic instability or the high cost of raw materials used to make many of these silicone hydrogels. While various solutions have proven somewhat successful for each of these deficiencies, there remains a need for silicone hydrogels that can be made from inexpensive commercially available monomers, and which have excellent wettability (without the need for surface modification), low modulus, good clarity, and desirable oxygen permeability.
Silicone hydrogels formulations containing polymeric wetting agents, such as poly(N-vinylpyrrolidone) (PVP) and acyclic polyamides have been disclosed. However, these polymers are quite large and require the use of special compatibilizing components, which need to be custom manufactured. Examples of compatibilizing components include 2-propenoic acid, 2-methyl-, 2-hydroxy-3-[3-[1,3,3,3-tetramethyl-1-[(trimethylsilyl)oxy]disiloxanyl]propoxy]propyl ester (SiGMA).
An alternative means of forming a wettable silicone hydrogel lens is to incorporate monomeric N-vinylpyrrolidone (NVP) into the monomer mix used to make the silicone hydrogel polymer, typically in amounts of about 25-55% (by weight) of the monomer mix. Such materials have been described in U.S. Pat. Nos. 4,136,250; 4,153,641; 4,260,725 and 6,867,245. The materials described in these references generally incorporate polyfunctional silicone monomers or macromers, that act as crosslinking agents, and thereby increase the modulus of the final polymer. U.S. Pat. No. 4,139,513 discloses that 2-propenoic acid, 2-methyl-, 2-hydroxy-3-[3-[1,3,3,3-tetramethyl-1-[(trimethylsilyl)oxy]disiloxanyl]propoxy]propyl ester (SiGMA) can be used to form lenses from formulations comprising NVP and HEMA. SiGMA is the only source of silicone disclosed. However, because of the relatively low silicone content in those monomers, desirable levels of oxygen permeability in the final polymers are difficult to achieve.
US 2010/0048847 discloses silicone hydrogels made from a blend of a monomethacryloxyalkyl polydimethylsiloxane methacrylate with about 52% NVP, HEMA and TRIS, and using a blend of ethanol and ethyl acetate as a diluent. The polymers disclosed are (to varying degrees) hazy, but it was disclosed in this application that the haziness could be reduced by the addition of at least about 1.5% methacrylic acid (MAA).
Addition of anionic monomers such as MAA can, however, cause hydrolytic instability in silicone hydrogels, as was disclosed in “The role of ionic hydrophilic monomers in silicone hydrogels for contact lens application”, Lai, Y., Valint, P., and Friends, G.; 213th ACS National Meeting, San Francisco, Apr. 13-17, 1997. For this reason, it remains desirable to form clear, hydrolytically stable, wettable (without surface treatment) silicone hydrogels with low moduli from a combination of a monomethacryloxyalkyl polydimethylsiloxane methacrylate such as mPDMS, and NVP.